Heterogeneous catalysts often consist of an active phase that is unstable in air. This is true of, for example, hydrotreatment catalysts (or refining reactions or hydrocarbon hydroconversion reactions) that contain the metal combinations CoMo, NiMo, NiW on an alumina and/or alumina-silica substrate, whereby said combinations contain an active phase such as metal sulfide. Another example consists of hydrogenation catalysts or steam-reforming catalysts with a nickel base that contain 10 to 70% by weight of Ni. These same solids are also used as sulfur traps. After the oxide NiO in the metal is reduced, the solid is air-sensitive and even pyrophoric. This is also true of systems with a copper base, such as copper-chromium for hydrogenation and copper-zinc for the conversion reaction of carbon monoxide (water gas shift) in the hydrogen units. There are also catalysts with an iron base in these same units (iron-chromium) or for the synthesis of ammonias that are used after reducing activation. In another category, there are catalysts for isomerizing n-butane or light naphtha with a platinum and strongly chlorinated alumina base of which the active phase, which is related to aluminum chloride, is highly sensitive to the ambient atmosphere (oxygen and water).
The practices developed by the industry take this constraint into account.
Thus, for hydrotreatment catalysts of the CoMo, NiMo, and NiW type on the preferably amorphous substrate, the active phase is produced only after the catalyst is loaded into the reactor itself, under pressure from hydrogen and a sulfurated compound. At the end of the cycle, the catalyst is removed from the reactor and brought into contact with the air without violent reactions occurring since the active phase is coated with compounds that contain carbon and limit contact with oxygen. Nevertheless, these catalysts may heat up somewhat during aeration, owing to oxidation of the sulfide phases, and certain specialists have tried to solve these problems. For example, the addition of polyaromatic compounds, Patent U.S. Pat. No. 4,912,071 by KASHIMA and CHIYODA, Japan will be cited; the addition of compounds such as amines, patents by Nippon Mining Company filed in 1977 exclusively in Japan.
For catalysts with a transition metal base, such as nickel, copper, or iron, the stage for reduction under hydrogen is also generally carried out in the unit. In some cases, the reduction is performed "off site" in a reactor that is separate from the reaction system, and the catalyst should then be passivated to avoid any risk of heating during transport, storage of the catalyst, and loading of the reactor. This passivation can be carried out in three different ways.
The most common is surface oxidation wish air that is diluted with an inert gas. This is carried out by raising, very slowly, the partial pressure of oxygen to limit the rise in temperature. This procedure makes it possible to create a surface layer of metal oxide that protects the reduced metal and thus makes it possible to handle the solid in air. The drawback is that it is again necessary to eliminate this oxide layer in the catalytic reactor, in general when hot. For example, if a catalyst with a nickel base when fresh is to be reduced at around 400.degree. C. in the unit, a catalyst that is reduced off-site and passivated by surface reoxidation should again be reduced in the unit at a temperature of 200-250.degree. C. This requires that there be suitable furnaces upstream from the reactor. Another drawback is the generation of H.sub.2 O corresponding to the elimination of the oxygen that is provided during the reoxidation stage. Another technique that is related to the latter is reoxidation/passivation by carbon dioxide. Finally, it is also possible to wet the catalyst with a liquid that prevents the oxygen from diffusing into the catalyst grain. The difficulty is then to load the reactor with a solid/liquid mixture or at best a wet solid and one whose grains stick together.
In the case of isomerizing catalysts with a chlorinated alumina base, there is no available passivation technique that can be reversible since O.sub.2 or H.sub.2 O irreversibly alter the active sites. The techniques that are employed are then to use very tight packaging for transport, such as, for example, metal drums with tight covers and systems for loading under inert gas that make it possible to limit contact between the solid and traces of air or moisture.